1. Field of the Invention
The present invention relates to an optical information reading and writing device using an optical information data recording medium.
More particularly, the present invention relates to an optical pickup device for focus control and/or tracking control of the optical information reading and writing device with the use of reflection rays emitted from the optical information data recording medium.
The present invention further relates to a method for detecting focus error of the optical system of the information data reading and writing device. The method can be used for controlling focus of the optical system for converging rays and irradiating them to an optical information data recording medium such as an optical disk, a prototype thereof and magneto-optical disk.
2. Description of the Related Art
An example of a system for focus control and/or tracking control in response to the reflection rays from the optical information data recording medium comprises a semiconductor laser element, a collimator lens for paralleling rays emitted from the semiconductor element, a beam splitter through which the parallel rays pass, an objective lens for converging the parallel rays and irradiating them to a magneto-optical disk on which optical information data is recorded. A part of the rays reflected by the disk propagates back to the beam splitter which reflects the rays and guides them to an optical signal detection system for detecting data signals from the magneto-optical disk.
The optical signal detection system comprises a halfwave plate, a condenser lens, a microprism detector comprising a microprism, a polarization beam splitter and an optical detector comprising a light receiving element which is divided to three parts. The reflection rays from the disk pass through the halfwave plate, the condenser lens and the microprism in the optical signal detection system. The reflection rays then either propagate further through the polarization beam splitter or are reflected by the polarization beam splitter. The rays which pass the polarization beam splitter are guided directly to the light receiving element. Whereas rays which are reflected by the polarization beam splitter are guided to the light receiving element through a mirror.
The focus error signals and magneto-optical data signals are obtained by comparing the optical amount of the light detected by the two parts of the light receiving element in accordance with for example a beam size method known per se, so as to control the focus of the magneto-optical disk and read the information data.
A second example of a system for focus control and/or tracking control in response to the reflection rays from the optical information data recording medium comprises a semiconductor laser element, a collimator lens for paralleling the optical rays emitted from the semiconductor laser element, a polarization beam splitter for reflecting the parallel rays, a quarter-wave plate and an objective lens for converging the rays and irradiating them to an optical disk. A part of the rays reflected by the optical disk surface propagates back toward the polarization beam splitter. The reflection rays pass the polarization beam splitter which guides the rays to an optical signal detection system for detecting information data from the optical disk.
In this optical signal detection system, the reflection rays from the optical disk are converged by a condenser lens and then either pass through or are reflected by a beam splitter. After that, the rays are detected by a light receiving element which is disposed on each of the optical paths for rays which pass through the beam splitter and rays which are reflected by the beam splitter. The focus error signals are obtained from the detection result of the two light receiving elements so as to control and adjust the focus of the optical disk.
With regard to the first example of the focus and tracking control system mentioned above, the optical signals are detected by the microprism detector of the optical signal detection system wherein the microprism and the detector are mounted on a same support plate. Therefore, it is difficult to individually adjust each position of the two optical spots, one being formed by rays which pass through the polarization beam splitter and the other being formed by rays which are reflected by the polarization beam splitter and the two spots being detected by a different optical detector, respectively.
Also, with regard to the second example of the focus and tracking control system mentioned above, the two light receiving elements are arranged perpendicular to each other in the optical signal detection system. Therefore, relatively large area is required for arranging the system, which impedes to realize a small and compact optical control system.
On the other hand, Japanese patent application Laying Open (KOKAI) No. 61230634 discloses an optical information reading and writing device using a diffraction grating.
In accordance with a first example of an optical information reading and writing device using a diffraction grating, a laser beam of rays emitted from a semiconductor laser element is made parallel by a collimator lens and irradiated to a polarization beam splitter. The diffraction direction of the polarization beam splitter is prearranged in parallel with the direction of the grooves thereof so that the incident light is diffracted toward a quarter-wave plate which converts the light to a circularly polarized light. The circularly polarized light is then converged by an objective lens and irradiated to an optical disk which is a medium for recording optical information data.
Also, a part of reflection rays from the surface of the optical disk propagates back to the quarter-wave plate which converts the light to a linearly polarized light. The linearly polarized light passes through the polarization beam splitter and is guided to a critical angle diffraction grating of an optical information detection system.
In this optical information detection system, the incident light is diffracted through two times of critical angle diffraction and a total reflection. After that, the diffracted light propagates to a light receiving element which is divided to four parts (segments). Focus error signals and/or track error signals are obtained from the difference of output signals between the four parts of the light receiving element.
A second example of the optical information reading and writing device using a diffraction grating means comprises a dual grating. In accordance with this second example, a laser beam of rays emitted from a semiconductor laser element is made parallel by a collimator lens and after that passes through two beam shaping prisms. The laser beam is then reflected by a beam splitter and converged by an objective lens and irradiated to a magneto-optical disk so that information data is recorded thereon.
Also, a part of the reflection light from the surface of the magneto-optical disk propagates back through the beam splitter and is guided to an optical information detection system. In the optical information detection system, the reflection beam is converged by a condenser lens and after that passes through the two gratings formed on the upper and lower surfaces of the dual grating element in either way of penetrating substantially straight therethrough or being diffracted thereby.
The zero-order light which penetrates substantially straight through the dual grating element is guided to a light receiving element which is divided to four parts (segments). Whereas the first-order light which is diffracted by the dual grating element is guided to another light receiving element which is divided to two parts.
A magneto-optical signal is obtained from the optical amount difference between the zero-order light and the first-order light. Also, a focus error signal is obtained from the zero-order light using a method of astigmatism. Further, a track error signal is obtained from the first-order light using a push-pull method.
With respect to the first example of the optical information reading and writing device using a diffraction grating means mentioned before, the device can be applied to a write-once optical disk system and a CD (compact disk) system, however, cannot be applied to a system for detecting signals from a magneto-optical disk. Also, the device of the first example involves a problem that the diffraction angle of the grating changes in response to change of wavelength of the light which passes through the grating.
Also, with regard to the second example of the optical information reading and writing device using a diffraction grating means mentioned above, a sufficient accuracy can not be obtained concerning the distance between the zero-order light and the first-order light so that the optical spot is often dislocated, which causes errors of detection and impairs the reliability of the result of signal detection.
In an optical system such as an optical pickup device or an optical disk prototype exposure system (or aligner), an exposure beam is converged and irradiated to the recording surface of the optical information data recording medium, i.e., an optical disk or an optical disk prototype. Such an optical system comprises a focus control system for controlling the focal point thereof so as to coincide the convergent point of the exposure beam and the disk surface to be irradiated. In order to reliably conduct such a focus control, it is necessary to detect the focus error, that is the dislocation between the convergent point of the exposure beam and the recording surface of the medium to be irradiated by the beam.
In order to detect such a focus error, for example, a double beam size method is used. In accordance with this method, the focus error is detected in such a way that a reflection beam of rays reflected from the surface of the optical information recording medium is divided to two convergent beams and that the sectional size of each of the divided two beams is detected so as to determine the dislocation of the focal point on the basis of the sectional sizes of the two beams.
Various method can be used for dividing the reflection beam from the information recording medium surface in the above-mentioned double beam size method for detecting the focus error. Examples of such a beam dividing method are a method using a single hologram (disclosed in Japanese patent application Laying Open (KOKAI) No. 58-121644), a method using parallel plane mirrors (disclosed in Japanese patent application Laying Open (KOKAI) No. 60-43234) and a method using a Wollaston prism (disclosed in Japanese patent application Laying Open (KOKAI) No. 62-26444).
However, with regard to the beam dividing method using a single hologram, there is a problem that the diffraction angle changes according as the wavelength of the beam changes, which causes detection errors of the focus dislocation. Therefore, a semiconductor laser element can not be used as a light source of the exposure beam since the wavelength of the semiconductor laser fluctuates.
Also, with regard to the beam dividing method using parallel plane mirrors, one of the divided two beams generates astigmatism due to the refractive function of the mirror system. Therefore, the reflection beam is not evenly divided, which causes errors of detecting the focus dislocation.
Also, with regard to the beam dividing method using a Wollaston prism, uneven astigmatism is generated in the two divided beams due to the double refraction of the prism system, which causes errors of detecting the focus dislocation.
Besides, in accordance with the double beam size method mentioned above, if the distance between the two convergent points of the divided beams is large, the size of the light receiving detector element have to be enlarged. Therefore, in order to realize a compact structure, there arises a problem that the range of detecting the focus dislocation becomes narrow.